1. Field of the Invention
The present invention relates to an electrophotographic image forming technique and, more particularly, to a developing method and developing device for electrophotographic image, and a printing device using the developing device.
2. Description of the Related Art
An electrophotographic image forming method is an image forming method adapted in copying machines, laser printers, and the like. In this electrophotographic image forming method, generally, uniform static charges are preliminarily given onto a photoconductive insulator layer, and the photoconductive insulator layer is irradiated with a light image, thereby partially removing the static charges to form an electrostatic latent image. Further, a fine powder called a developer (toner) is adhered to the part having the residual static charges on the photoconductive insulator layer to visualize the latent image. The resulting toner image is then formed (developed) on and fixed to a recording sheet, thereby obtaining a printed matter.
The developing method (forming method) for electrophotographic image is roughly classified to a two-component developing method using a two-component developer formed of magnetic carrier and nonmagnetic or magnetic toner and a one-component developing method using a one-component developer formed of only magnetic or nonmagnetic toner. The one-component developing method is further classified to a magnetic one-component developing method using magnetic toner, and a nonmagnetic one-component developing method using nonmagnetic toner.
In this specification, the nonmagnetic one-component developing method using nonmagnetic one-component toner (developer) and such a nonmagnetic one-component developing toner will be mainly described. However, the present invention is never limited to such nonmagnetic one-component developing method and nonmagnetic one-component developing toner, and can be extensively applied to developing methods for electrophotographic image using various toners, and such toners for developing electrophotographic images.
A conventional electrophotographic image forming technique (image forming process) is described below.
One example of a conventional nonmagnetic one-component developing device is schematically shown in FIG. 1. Such a developing device is disclosed in detail, for example, in Japanese Unexamined Patent Publication (Kokai) No. 60-229057, Japanese Unexamined Patent Publication (Kokai) No. 61-42672 (corresponding U.S. Pat. No. 4,930,438), or the like.
As shown in FIG. 1, the conventional nonmagnetic one-component developing device (developing unit) 7 is provided with a storing means (toner tank, etc.) 1 for storing a developer (toner) 8; a developer supply mechanism (stirring paddle) 2 for conveying the toner along a circulating route; a developer carrier (developing roller, etc.) 3 for conveying the toner along a preset circulating route including a developing area; and a roller-like developer recovering means (recovery roller, etc.) 4 which is provided to make contact with the developing roller 3 and has a flexible material adhered to the surface part. Further, the developing unit is provided with a developer restricting element (restricting blade: toner restricting element) 5 for restricting the thickness of toner on the developing roller 3 and a photoconductive insulator (photosensitive drum, etc.) 6 for forming and holding an electrostatic latent image, which is arranged opposite to the developing roller 3 to be contactable thereto. The developing roller 3 is constituted so as to convey the toner supported on the developing roller 3 to the opposed photoconductive insulator 6 by rotation. The developing unit 7 having the toner tank 1, the stirring paddle 2, the developing roller 3, the recover roller 4 and the restricting blade 5 is replaced by a new one after printing (developing) a prescribed number of sheets.
The developing process is then described in detail in reference to FIG. 1.
The toner (developer) 8 is conveyed from the toner tank 1 onto the developing roller 3 via the stirring paddle 2.
The toner supplied onto the developing roller 3 reaches the restricting blade 5 by the rotation of the developing roller 3, and only a fixed amount guided depending on the clearance between the developing roller 3 and the restricting blade 5, the materials thereof or the like is supplied to the photosensitive drum 6. At this time, the toner is electrified to a desired charge by being strongly rubbed with the restricting blade 5 or receiving the charge injection of a potential applied, as occasion demands, to the developing roller 3 or restricting blade 5.
When the developing roller 3 is then opposed to the photosensitive drum 6, the toner on the developing roller 3 is transferred onto the photosensitive drum 6 according to the electrostatic latent image potential on the photosensitive drum 6 by use of an electric attractive force or repulsive force such as the potential (developing bias potential) applied to the developing roller 3, the electrified potential of the toner, or the electrostatic latent image potential formed on the photosensitive drum 6 as driving force to visualize the electrostatic latent image, whereby the developing is performed.
The toner which was not transferred to the photosensitive drum 6 in the developing by the developing roller 3 is removed by the potential difference (recovering bias potential) between the developing roller 3 and the recovery roller 4 or a mechanical friction (peeling force) when the developing roller 3 is further rotated and opposed to the recovery roller 4, and the electric history on the developing roller 3 is also erased.
As the toner, resin fine particles having average grain sizes of about 5 to 15 μm, which contain a natural or synthetic thermoplastic polymer resin (binder resin) having a weight average molecular weight of about several thousands to hundreds of thousands, a wax, a coloring agent, and, as occasion demands, a charge controlling agent or the like are generally used.
In the conventional developing unit, the toner is physically and electrostatically supplied from the recovery roller (reset roller) 4 to the developing roller 3 to perform a developing on the photosensitive drum 6, and the toner left on the developing roller 3 is then recovered by the recovery roller 4. In such a conventional developing unit, however, if the toner is still left on the developing roller 3 beyond recovery in the recovery of the residual toner on the developing roller 3 by the recovery roller 4, the left toner is repeatedly used again in the developing process, which leads to a printing failure or the filming or contamination of a functional member such as roller. The filming or contamination of the functional member consequently causes a reduction in life of the developing unit.
The developing roller 3, the recovery roller 4, and the restricting blade (toner restricting element) 5 are important parts which are repeatedly used for developing, and influence on electric characteristics such as carrying property of toner, frictional electrification amount, developing toner amount, and developing bias, and recovery bias potential. Therefore, these parts need to keep regularly stable physical and chemical characteristics during the drive of a printing device using the electrophotographic image forming method.
The disruption of the balance of these characteristics triggers a printing failure such as fog, fading, or after image. Therefore, in the electrophotographic image forming method using one-component developing method, for example, the need of replacing the developing unit arises when these members cannot keep desired characteristics because of the wear by repeated use or the like. The frequent replacement of the developing unit extremely disadvantageously brings about a rise of running cost of the printing device.
In an unused developing unit, on the other hand, each member shows physical and chemical characteristics derived from the constituting material of each member because it has not suffered a strong stress with toner yet. However, when this developing unit is used for printing, it suffers a strong physical stress with the toner.
Since the toner is mainly made up of a thermoplastic resin, as described above, and an energy-saved fixing has been strongly desired in recent years, the characteristics of the toner are becoming soft. Therefore, if the surface of the developing unit component is thinly filmed with the stressed toner just after the start of printing, the physical and chemical characteristics shown by each component are changed from the characteristic values derived from the constituting material to the values influenced by the thin filming with the toner. This change in physical property value, as a matter of course, inconveniently causes a change in printing characteristics.
As a means to cope with this problem, for example, it is adapted to subject a produced new developing unit to a test print of a fixed number of sheets, thereby thinly filming the surfaces of components with toner so that the physical and chemical characteristics shown by the components reach a steady state followed by shipping. However, since the printing of thousands sheets or more is often required in order to prevent the fluctuation of printing characteristics, the substantial life of the developing unit is shortened by the printing number of sheets in the test printing, and an increase in manufacturing cost further arises because the test printing involves a complicated work.
As another solution, it is also adapted to use a fragile material for each component, and successively peel, even if thinly filmed with toner, the thinly filmed outermost surface by the friction between members, thereby regularly exposing a fresh face to keep the physical and chemical characteristics peculiar to the components. However, this method involves the factor of shortening the replacing period of the developing unit, and cannot be said to be preferable.
Further, in a related art disclosed in Japanese Unexamined Patent Publication (Kokai) No. 63-276064, the difference between volume average grain size and number average grain size is minimized (a toner with minimized fine powder amount and sharp grain size distribution is regulated). This method is good for the effects to fog, low density, history after image and the like, but unsatisfactory in yield and cost. It further has the problem of reduction in electrification or fogging of background part that may be encountered when the fine powder amount is increased in continuous printing.
Further, a related art disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-22138 is effective for fog of background part, but has problems of lowered density, history after image and the like in an initial stage of printing. A related art disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-240925 (corresponding to U.S. Pat. No. 5,731,122) is similarly effective for history after image (positive memory, negative memory), but has problems of lowered density in an initial stage of printing and increased fluctuation of density in continuous printing.